I'm also trying to replace a Satchwell TLX 2259 with a Sunvic TLX 7501.

On my set up I have 3 wires coming out of the wall. An earth attached to the back plate, 1 Red and 1 Black each attached to a terminal on thermostat. The difference for me is that there is an additional electronic component (with markings: Z5U, 01Z, IKV) linking the 2 terminals Red and Black.

I've tried simply connecting Red to 3 and Black to 2 on the 7501, but it doesn't work!!!

To add to this (I have just joined and fully intend to use this forum properly) I have a satchwell tlx2259 and want to replace with a sunvic 7501. The issue I have is that the current config on the Satwell is Blue wire-1, Brown wire-3 and the yellow/green is made safe.